The cellular communications system known as LTE, Long Term Evolution, is a radio access technology for packet-switched services. In the uplink, i.e. traffic from users in a cell to the controlling node, the eNodeB of the cell, all users in the cell share the same physical channel for user data transmission.
The eNodeB is responsible for allocating or scheduling physical channel resources such as, for example, transmission times and frequencies to the users in a cell in order to allow the users (UEs/User Equipments) to transmit data in the uplink.
The scheduling of the physical channel resources to users in LTE should be such that the QoS, Quality of Service, requirements of the individual UEs are fulfilled.
The physical layer in the uplink of LTE is based on time slotted Single-Carrier Frequency Division Multiple Access, SC-FDMA. The single carrier transmission for a single user is based on OFDM, Orthogonal Frequency Division Multiplexing. A time slot, here also referred to as a subframe, has a duration of 1 ms and is divided into 12 or 14 OFDM symbols, depending on the system configuration.
One OFDM symbol comprises a number of subcarriers in the frequency domain, depending on the channel bandwidth. One subcarrier in one OFDM symbol can carry one modulation symbol. For each subframe, the eNodeB makes a scheduling decision in order to determine which UEs that will be allowed to transmit using which resources. The smallest resource unit that can be assigned to a UE is a so called scheduling element, which is a time-frequency unit of the size 180 kHz×1 ms. A user can be assigned a number of scheduling elements in a subframe.
The eNodeB's scheduling decision for a subframe is transmitted downlink on a so called L1/L2 control channel to the UEs in the cell before the uplink data transmissions are made. The messages transmitted on the L1/L2 control channel contain a scheduling grant for each UE that is assigned one or more scheduling elements in the uplink subframe. The uplink grant contains such parameters as the scheduling element resource allocation, the transport-block size, the modulation scheme, and a transmission power adjustment that the UE needs to use.
The downlink L1/L2 control channel is a scarce resource, and therefore the number of scheduling grants that can be transmitted in a subframe is limited.
To aid the scheduling decision-making in eNodeB, the UEs in the cell of the eNodeB periodically transmit buffer status reports and uplink channel quality information reports to the eNodeB. The buffer status report contains information about the number of bytes that the user would like to transmit in the uplink.
Synchronous HARQ is used in the LTE uplink in order to improve the robustness against transmission errors. After a UE has transmitted its user data and the eNodeB has decoded the transmitted user data, a single ACK/NACK bit is sent to the UE. Upon the reception of a NACK, the UE retransmits the user data. The use of synchronous HARQ means that the time between transmission and retransmission is fixed, and known to both the UE and eNodeB, with the benefit that there is no need to send a scheduling grant for the retransmission on the downlink control channel; instead the information in the initial scheduling grant is reused.
Since LTE at present is an emerging technology, there have not as yet been any commercial solutions implemented for the uplink scheduling in the eNodeB. However, such a solution should take into account at least the following system constraints:                The eNodeB can only allocate scheduling elements which are consecutive in frequency to an UE,        The time between uplink transmission and retransmission is fixed. This means that retransmissions must be transmitted a fixed number of subframes after the initial transmission.        The downlink L1/L2 control channel resource is a scarce resource. The number of uplink grants that can be transmitted in a downlink subframe depends on the number of OFDM symbols used for the downlink L1/L2 control channel and the channel quality of the UEs that are the intended receivers of the scheduling grants,        It is not necessary to transmit a scheduling grant for a retransmission that uses the same scheduling elements, transport block size and modulation scheme as the initial (or previous) transmission,        If it is necessary or beneficial, eNodeB may change the set of scheduling elements used for a retransmission by sending a scheduling grant for the retransmission, thereby indicating the scheduling elements that should be used for retransmission,        The UEs are in many cases power limited.        